1. Field of the Invention
The present invention relates to a silicon carbide substrate, a semiconductor device, and methods for manufacturing them. More particularly, the present invention relates to a silicon carbide substrate capable of reducing on-resistance and improving yield of semiconductor devices, a semiconductor device using the substrate, and methods for manufacturing them.
2. Description of the Background Art
In recent years, in order to achieve high breakdown voltage, low loss, and utilization of semiconductor devices under a high temperature environment, silicon carbide (SiC) has begun to be adopted as a material for a semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously. In addition, a silicon carbide substrate is more excellent in heat conductivity than a substrate made of a nitride semiconductor such as a gallium nitride substrate, and thus it is excellent as a substrate of a semiconductor device for high power application for which high voltage and large current are required.
In order to manufacture a high-performance semiconductor device adopting silicon carbide as its material, it is effective to employ a process of preparing a substrate made of silicon carbide (silicon carbide substrate), and forming an epitaxial growth layer made of SiC on the silicon carbide substrate. Further, when manufacturing, for example, a vertical type power device using the silicon carbide substrate, on-resistance of the device can be reduced by reducing resistivity of the substrate in the thickness direction thereof as much as possible. Further, in order to reduce the resistivity of the substrate in the thickness direction thereof, for example, there can be employed a method of introducing an impurity, which is an n type dopant such as nitrogen, into the substrate at a high concentration (for example, see R. C. GLASS et al., “SiC Seeded Crystal Growth”, Phys. stat. sol. (b), 1997, 202, pp. 149-162 (Non-Patent Literature 1)). In addition, influence of an impurity present in a surface of a Group III nitride substrate having a crystal structure different from that of silicon carbide has been studied (see Japanese Patent Laying-Open No. 2011-77381 (Patent Literature 1), Japanese Patent Laying-Open No. 2010-163307 (Patent Literature 2)).
However, even if a semiconductor device is manufactured using a silicon carbide substrate having a low resistivity, there may occur a problem that on-resistance of the semiconductor device is increased and causes a reduction in yield.